The demand for weight reduction in the automotive industry has led to the development and implementation of lightweight materials, and related manufacturing processes and tools. The growing concern for occupant safety also leads to the adoption of materials which improve the integrity of the vehicle during a crash while also improving the energy absorption.
A process known as Hot Forming Die Quenching (HFDQ) uses boron steel sheets to create stamped components with Ultra High Strength Steel (UHSS) properties, with tensile strengths up to 1,500 MPa or even more. The increase in strength allows for a thinner gauge material to be used, which results in weight savings over conventionally cold stamped mild steel components.
Typical vehicle components that may be manufactured using the HFDQ process include: door beams, bumper beams, cross/side members, A/B pillar reinforcements, and waist rail reinforcements.
Hot forming of boron steels is becoming increasingly popular in the automotive industry due to their excellent strength and formability. Many structural components that were traditionally cold formed from mild steel are thus being replaced with hot formed equivalents that offer a significant increase in strength. This allows for reductions in material thickness (and thus weight) while maintaining the same strength. However, hot formed components offer very low levels of ductility and energy absorption in the as-formed condition.
In order to improve the ductility and energy absorption in specific areas of a component such as a beam, it is known to introduce softer regions within the same component. This improves ductility locally while maintaining the required high strength overall. By locally tailoring the microstructure and mechanical properties of certain structural components such that they comprise regions with very high strength (very hard) and regions with increased ductility (softer), it may be possible to improve their overall energy absorption and maintain their structural integrity during a crash situation and also reduce their overall weight. Such soft zones may also advantageously change the kinematic behaviour in case of a collapse of a component under an impact.
Known methods of creating regions with increased ductility (“softzones” or “soft zones”) in vehicle structural components involve the provision of tools comprising a pair of complementary upper and lower die units, each of the units having separate die elements (steel blocks). The die elements may be designed to work at different temperatures, in order to have different cooling rates in different zones of the part being formed during the quenching process, and thereby resulting in different material properties in the final product e.g. soft areas. E.g. one die element may be cooled in order to quench the corresponding area of the component being manufactured at high cooling rates and by reducing the temperature of the component rapidly. Another neighbouring die element may be heated in order to ensure that the corresponding portion of the component being manufactured cools down at a lower cooling rate, and thus remaining at higher temperatures than the rest of the component when it leaves the die.
In order to heat the die elements, electrical heaters located inside the die elements and/or channels with hot liquids e.g. oils may be used.
One problem related with this sort of heating may be that it is necessary to machine the die elements in order to allocate the electrical heaters and/or the channels with hot liquids. Machining the die elements may be costly and sometimes difficult to perform, especially if the geometrical shape of the die elements is complex. Reliability is also an important factor. In the channels with hot liquid, hot liquid leakages might occur and repair can take time. In the electrical heaters, malfunctioning heaters might be difficult to detect and repair.
Moreover, the temperature of the die should preferably be as homogenous as possible in order to create an accurate soft zone. In the above commented solutions, the heat focus may be at a point or along a line, and thus the die elements surface are not heated uniformly. This could lead to different material properties in the same portion of the structural component.
Additionally, in the channels with the hot liquid solution, hot liquid leakages may occur. This can lead to an increase of the risk for the operator especially if the operator may be standing near the leakage. Furthermore, the repair can take time and, in some cases, a new die element with the machined channels may be required.
DE102005032113 discloses an apparatus for thermally deforming and partially hardening a component in a mold of at least two parts, between which the component, at its hardening temperature or above, is compressed to the mold contour by a press, each mold part is subdivided into segments separated by thermal insulation. The segments are adjustable to different, controlled temperatures for adjusting the component to different temperatures during pressing.
US2014260493 is related to a hot stamping mold apparatus. This apparatus may include a bottom part equipped on a bolster and a top part equipped on a slider, wherein the bottom part and the top part each include a cooling mold including a plurality of coolant chambers formed therein, a heating mold installed at a side of the cooling mold to form a formed surface together with the cooling mold and provided with a heating cartridge installed at a side of the heating mold.
DE102004026762 discloses a press tool for metal sheets includes a heated section with integral electric heating elements for areas of large press changes. The heated section is thermally insulated from the rest of the tool system by a ceramic layer integrated into the tool. The heated tool section can be made of thermally conducting ceramic.
FR2927828 discloses a thermoforming mold for forming and cooling a steel part from a blank, the tool comprising: at least one punch and at least one die the punch and the die each comprising: at least a first portion (21, 31) corresponding to a hot zone (11) of the stamping tool and at least one second portion (22, 32) corresponding to a cold zone (12) of the stamping tool in the cold zone, the second portion of the punch and the second part of the die coming into contact with the blank when the tool is closed
It is an object of the present disclosure to provide improved tools for manufacturing hot-formed vehicle structural components with regions of high strength and other regions of increased ductility (soft zones).